The invention relates to a method and apparatus for providing a video clamp having more than 50 dB low frequency noise rejection.
One of the major problems connected with the transmission of a television signal is the exceptionally wide video band of frequencies involved. In the transmission of the television signals of video frequency, the problem is further complicated because the lower limit of the frequency range extends literally to zero frequency. Therefore, to provide low frequency noise rejections, video clamps are added to the television receiver which should remove under ideal conditions the low frequency noise without degradation of the video signal.
In general, a video clamp is used to remove deviations of a designated portion of a video signal, such as blanking level or synchronization tip level from an arbitrary reference level. In an article published in 1950, Vol. 69 by the AIEE Transactions, there were described several video clamps and their applications. Among these are low frequency compensation circuits, DC restorer circuit, diode clamp type circuits, series capacitor clamper circuits and feedback clamper circuit, as well as other configurations. Although this article does not use the present day semi-conductor capabilities, it does provide an overview of the basic circuit arrangements that are used in the theory and application of clamp circuits.
In a present day video transmission system, which may even include a satellite relay station as shown in FIGS. 1 and 2, there is an additional low frequency noise purposely introduced in the transmission of video signals which is used to prevent a high concentration of energy around the carrier frequency. This requirement is not only to prevent the undesirable consequence of having a high concentration of energy around the carrier frequency, but it is also a requirement of the Federal Communications Commission's specification.
For an understanding of the operation of a satellite communications system as well as the technique used to prevent a high concentration of energy at the carrier frequency, reference should be made to FIG. 1 in which there is a source of the video signal 1 that can be a device such as a video camera or tape player. The output of the source of the video signal is provided on conductor 3 which connects the composite video signal generated by source 1 to the energy dispersal generator (EDG) 5. The energy dispersal generator (EDG) 5 provides a 30 hertz triangular waveform, which is summed with the composite video signal and is present on the output of the energy dispersal generator that is connected to conductor 7. The 30 hertz dispersal signal will spread the energy at carrier frequency over a wideband thus avoiding a peak concentration of energy at the carrier frequency. The output of the energy dispersal generator is applied to the FM modulator 9.
The output of the FM modulator is placed on conductor 11 for upconversion by mixing the signal present on conductor 11 with a local oscillator signal. The mixing process is accomplished by the apparatus that includes a mixer means 13 which is connected to an oscillator 17 by means of a conductor 15. The output of the mixer means 13 is provided on conductor 19. At this point, the frequency modulated composite video signal has been upconverted by being mixed with a 1170 megahertz source provided by oscillator 17. The output of the mixer means 13 is connected to the intermediate frequency (IF) section 21 by conductor 19.
The intermediate frequency section provides a 1100 megahertz signal on conductor 23 which again is passed through another mixer stage 25 where it is mixed with the local oscillator 29 which is connected to mixer means 25 by means of conductor 27. The output of the mixer 25 upconverts the 1100 megahertz signal to a frequency that can be between 5 and 6 gigahertz for transmission by antenna 33, which is connected to the mixer means 25 by conductor 31, to a satellite repeater station 35. The signal that the satellite repeater station 35 receives is a 5 to 6 gigahertz signal modulated with a composite video and audio components which have a bandwidth from 0 up to .perspectiveto.4.5 megahertz.
In addition to the energy dispersal input noise, there is also a possibility that the signal is imposed upon a low frequency waveform that occurs when the picture goes from a dark background picture to a light background and back to a dark background. This low frequency noise is generally referred to as "bounce" and has a frequency of around 10 hertz. In addition to the bounce and energy dispersal noise, there is also the ever present sixty hertz noise on the signal.
The satellite repeater station as shown in FIG. 2 retransmits the signal to a receiving antanna 37. It is applied to a low noise amplifier (LNA) 41 by means of conductor 39. The low noise amplifier 41 provides the noise figure for the signal that is provided on conductor 43 which connects it to mixer means 45. Mixer means 45 downconverts the received signal from a 4 gigahertz signal which is the frequency range of transmitted information from the satellite repeater station 35 to around 1100 megahertz. Downconversion is accomplished by local oscillator 49 which is connected to mixer 45 by means of conductor 47. The downconverted signal is present on conductor 51 and is applied to the first intermediate frequency (IF) stage, the output of which is downconverted again by mixer means 57 which mixes a local oscillator signal generated by local oscillator 59 and connected to mixer means 57 by means of conductor 61. The mixing frequency is normally around 1170 megahertz so that the output frequency on conductor 63 is 70 megahertz. Conductor 63 connects the 70 megahertz signal to discriminator 65 which provides the composite video signal upon conductor 67. At this stage, there is present a composite video signal that is superimposed upon a noise signal that is a composite signal having the thirty cycle triangular waveform plus sixty cycle noise as well as the possible "bounce" noise. This is applied to the video clamp 69 which must be able to remove all of the enumerated noise signal from the composite video. The output of the video clamp is present on conductor 71 which connects the composite video signal to a (video/audio) separator 73 that normally includes different filters. The audio at this point is removed and is present on conductor 75 and connected to audio amplifier 77, the output on conductor 79 is the audio signal and the video is present upon conductor 81 which is connected to video amplifier 83 and the output is provided upon conductor 85 for processing by the video section of the receiver.
It is obvious from the above discussion that the video clamp 69 of FIG. 2 must have very high low frequency noise rejection. It has been found that with the receiver shown in FIG. 2 that the video clamp must have a minimum of 50 dB low frequency noise rejection. The prior art video clamps were not able in practice to achieve this noise rejection.
A clamp circuit that reduces low frequency noise on a wide-band signal such as a video signal by more than 50 dB is disclosed and includes an input amplifier for obtaining a difference signal that is the algebraic summation between an input signal having periodically spaced synchronization pulses and a reference signal. The reference signal is generated by sampled data techniques and includes a means for removing the synchronization pulses from the signal. After passing the synchronization pulse through a pulse shaper, they are used to operate a sample and hold device which samples the output of the difference amplifier during the synchronization periods and the sampled output, after being passed through a low-pass filter, is used as the reference signal to the difference amplifier which subtracts the reference signal from the wide-band signal. An additional amount of low frequency rejection is provided by passing the difference signal through a switching clamp circuit.
In addition, there is shown a real time sync separator for removing the sync pulses from the received video signal, as well as a regenerated pulse generator and shaper.
The switching circuit for the sample and hold circuit is a diode bridge that is provided in detail with the associated circuitry.
Many other objects and purposes of the invention will be clear from the following detailed descriptions of the drawings.